The growing increase in the number of items of airbag equipment fitted in vehicles has allowed a great deal of information to be gathered regarding the operation of airbags, and this has made a great contribution to improving them. One of these improvements consists in subjecting the operation of the airbag to an intelligent control unit which is fed by various detectors so as to adapt the deployment of the airbag to certain circumstances.
Thus, for example, document DE-4237072 proposes a system for detecting whether the passenger seat is occupied so as to disable this airbag when this seat is not occupied in order not to have to replace the airbag when the vehicle is repairable after an impact which has triggered the deployment of the airbag.
Documents LU-88 547 and 88 588 propose a system for detecting whether the passenger seat is occupied by an auxiliary child seat and for detecting the orientation of this auxiliary seat in order to disable the airbag when this seat is facing in the rearwards direction, in order to prevent a child from being thrown violently towards the rear of the vehicle under the effect of the deployment of the airbag.
What is more, new gas generators are on the drawing board so that in the future it will be possible to have a multi-level operation and a gradual and more subtle than all or nothing deployment, especially in order to take account of the inertial mass of the passenger, which can be obtained from the information supplied consists of a pressure sensor of the FSR type.
It has also been observed that the deployment of an airbag can be very dangerous for a passenger when this passenger is occupying a position other than his so-called normal position. It should be realized that an airbag deploys in the form of a mushroom with a sudden longitudinal spurt followed by lateral inflation. Now, if the passenger's head, for example, when the passenger is looking for something in the storage binnacle, is too close to the airbag cassette, and therefore in the field of deployment of the airbag, particularly in the path of the initial spurt, the effect of the airbag may be more harmful than beneficial.
This is why a method for controlling the deployment of an airbag as a function of the position of the seat occupant has already been proposed. Document GB-A-2,236,419 describes such a method. The occupant's position is determined by telemetry measurements based either on the occupant's body cutting a beam or by the journey time for a wave reflected off the occupant.
Document WO-A-94/22693 also describes a method of this kind which employs radar waves to reconstruct an image of the occupant.
These measurements, based on reflection, may, however, be made false by an object that the occupant is holding in his hands, for example a book or a newspaper.
Methods based on reflection off marks provided on the seatbelt are also known. Such methods are known from Research Disclosure, 1994, January, No. 357, Emsworth GB, p. 50 or from U.S. Pat. No. 3,748,639. These methods are used to check whether the seatbelt is being worn or to determine how far it is unwound.